Shock absorbent strut for aircraft



Feb 8, 1938- .1. H, oNloNs ET Al. 2,107,494

SHOCK ABSORBENY STRUT FOR AIRCRAFT Feb. 8, 1938'. 4 J; HI ONIONS ET AL 2,107,494

SHOCK ABSORBENT STRUT FOR AIRCRAFT Patented Feb. 8, 1938 PATENT OFFICE r 2,107,494. snoek ABsoaaEN'r s'raU'r Foa AIRCRAFT John Henry Onions and Peter Warborn Thornhill, Leamington Spa, England; said Thornhill assignor to said Onions Application August 19, 1936, Serial No. 96,872

In Great Britain October 15, 1935 's 9 Giannal (cl. 267-64) This invention relates to resilient struts, particularly for aircraft, of the kind in which a plunger tube is slidably mounted within a cylinder tube, each being closed at a position remote from its inner end to provide an internal space of variable volume `containinga liquidsuch as oil, and gas under pressure such as air, the liquid during relative reciprocationof the tubes in one or both directions being `caused to pass through one or more restricted openings for damping this movement of the tubes, while the compressedgas provides the whole of the resilience normally required, the strut, of course, being capable of carrying a sustained normal axialn load without becoming completely closed.

It is the primary object of the present invention to provide an improved construction of strut having characteristics which render it particularly satisfactory for incorporation in the landing gear of aircraft, for example, a low value of static friction coupled with high dynamic friction, particularly during an extension or rebound movement, the frictional load during such movement being widely distributed and being taken to some extent by the pressure of the plunger packing against the cylinder wall, as well as by the# resistance of the liquid passing through the restricted opening or openings.

In a resilient strut of the kind referred to above having a plurality of resilient packing means carried by the inner end of the plunger tube, ac-

cording to one feature of the invention the peripheral portions of two adjacent packing rings upon the plunger are both inclined axially towards one another, and preferably the intervening `space is placed in'communicatlon with the liquid within the strut so that the fluidpressure thus created in said space tends to press said rings against the interior of the cylinder tube.

It also prevents air entering on extension of the strut and it provides lubrication to the plunger head and packings. s A Y The invention further provides a resilient strut of the kind referred to having the inner end of the plunger tube closed by a plunger head which is formed with a longitudinal passage"controlled by a flutter valve for regulating the damping effect, the exterior` of the plunger head being fitted with a plurality of packing rings co-operating with the internalv surface of the cylinder tube, and the invention is characterized by the fact that the space between two of the said rings is connected with the interior of the plunger head by means of one ormore passages which terminate against the edge of the flutter valve -so that the passage or passages are placed in direct communication with one side of the flutter valve or the other side according to the position occupied by said valve whereby the said space is incommunication with whichever side of the head is subject to the higher pressure. If desired, at least two of the packing rings are substantially frustoconical in general form, their peripheral portions being shaped to provide surfaces co-acting with the interior of the cylinder, both of said periph'- eral portions being directed obliquely towards one another. s

The preferred construction of resilient strut according-to the invention has at least one of the tubes turned inwardly at its inner end, i. e. that end which overlaps the other tube, this turned-in portion enabling a plunger head to be easily secured in the case of theinner tube and permitting an improved form of limiting stop to be provided in the case of the outer tube. Thus the invention provides a resilient strut in which the plunger tube isturned inwardly at its inner end and is clamped axially to a plunger head, and the cylinder tube is also turned inwardly at its inner end to prevent excessive extension of the strut, one or more sleeves of anti-friction material conveniently being interposed between the plunger tube and the cylinder. If desired, a sleeve composed of anti-friction material is carried by the plunger tube but is slidably mounted in relation thereto, and an annular buffer of resilient material is interposed between this sleeve and an abutment carried by the plunger tube, said sleeve serving as a stop by which the maximum extension of the strut is limited, and the butler being arranged to prevent shock when this limit is reached.

The invention is illustrated by way of example in the accompanying drawings, in which:-

Figure l is a sectional elevation drawn diagrammatically showing the internal arrangement of one form of strut;

Figure 2 is a side elevation to a reduced scaleA showing a strut of normal proportions constructed on the lines indicated in Figure i;

Figure 3 isa fragmentary sectional elevation showing the details of the plunger head of the strut;

Figure 4 is a diagrammatic sectional elevation showing a modified method of construction;

and

Figure 5 is a sectional plan on the line i, 5 of Figure 4.

In the rst example shown in Figures 1 to 3 the principal parts of the strut consist of an 7 en of the tube lo being ntted with a bushjta within which a plunger tube is slidably mounted', a soft packing ring I conveniently beingtted in the bush I3 to preventthe entrance of dust or other foreign matter into the cylinder tube I0.

,The inner end of the plunger tube I4, i. e. that end which is overlapped by the inner end of the cylinder tube Ill, carries a plunger head I6 which is formed at I1 with an axial recess fitting snugly over the outside of the tube I4, the end I8 of the latter being turnedA inwardly as shown and being clamped in position by a series of bolts I9 acting upon a clamping ring 20. By this means an extremely firm yet easily constructed joint is obtained.r v

At its outer end the plunger tube I4 is closed by a plug member 2|, a lug 22 formed upon which serves as a convenient anchorage for the upper end of the strut. kThe plug member 2I is fitted ,with a pair of longitudinal tubes 23 and 24 communicating respectively with a pair of passages closed by removable plugs 25 and 26, while an air valve 21 also communicates with the interiorof the plunger tube I4 by way of the plug member 'In order tolimit the inward movement of the plunger tube I4 and to prevent shock when this limit is reached, anannular rubber or-other rel, silient buifer 28 normally rests upon the plugy member I I and. is engaged by thewplunger head Iii)V while a similar buifer 29 surrounding the plunger tube I4 is supported by a shoulder-30 formed upon distance tube 3I bearing at its other end upon' he plunger head I6. Thus, when the maximum pennissible extension has been obtained, 'further movement is arrested by the buffer 29 engaging with theunder surface 32 of the bush I3.

Reverting to the plunger head I6 which :isl concerned mostly with the \present invention, it will be lseen that the externalcurved surface ofthis is provided with three packing rings y33, 34 and 35, each' being of substantially'frusto-conical shape and conveniently being composed of flexible material, such as rubber. For their accommodation, correspondingly shaped grooves liaving-suizlstan-y tially parallel'sid'e walls are provided, thesewalls being indicated for example at 36 and 31 inthe distance separating the said walls 3Gand 31 isf-.34 and 35 As a-resulmncreased retarding mc just a little in excess of the corresponding` thickness of the packing ring, in order that liquidunder pressure can enter the groove and act upon said ring inL order to press it outwardly. into firmcontact with ther interior of the cylinderjubel Il. The

34 and 35"is,\ofloure, shaped to conform to the cylindrical surface of "thegtube I0, and this provides a lip 38 in each case which is readily acted upon by iluid pressureto produce-a tlghtlsliding contact, the frictional resistance beingapproximately proportional to the Huid pressure exerted upon said lip. lThis action isl utilized as will be hereinafter explained, but it will be notedthat the ble of being covered by\a plate 44 disposed within the recess 4I and constituting a flutter valve. The

drilled in the centre of the wall' 42. A space V41 beneath the plunger head. I6 is occupied completely by liquid, such as oil, and this extends'into the plunger tube up to the level indicated at 48, while 10 the space 49 above the, liquid is occupied by compressed gas. When, therefore, the plunger tube I 41s forced into the cylinder tube I0, under the action 'say of the aircraft' alighting upon the ground, the liquid in the space 41 is put under 15 pressure forcing it to flow through the holes 43, lifting the flutter valve 44 and forcing up to the clamping ring 20. The fluid is permitted to pass freely through the comparatively large area of the holes 43 but receives a predetermined but 20 relatively small amount of obstruction at the hole 45, thus causing the level 48 to rise and further compress thefgas in the space 49. During this movement it willl be noted that there is a difference in pressure ybetween the liquid in the space 25 41` and that within the plunger tube I4.,l and, therefore, the flutter valve 44 is maintained in -its raised position so' that the space 39 between the rings 34 and v3.5 is always in communication with the space 4I and subject to the pressure .30 therein,\ l, i il 'I'hus lthe higher .pressure -in the spacep4I is caused to act upon the rings 34 and '35, which woulditend tov force both of them against the cylinder vvall; but the ring 35 is subject, on; its 35 cuter surface, to a balancing pressure from the space 41l and therefore on this stroke it does not exert any substantial amount of pressure on the cylinder wall, thus avoiding unnecessary .frictionl During vthe rebound movement, however, when\40 the plunger tube I4 tends to become withdrawn from the cylinder tube I0 the flutter valve 44 will be forced on to the wall 42, and as a consequence the liquid from the plunger tube'i4 has to pass through a very constricted passageway consti- 45 tuted by the hole 46. As a result-a'considerable diiference in pressureis created between the liquid in the plunger tube I4 and that in the space 41, and the higher pressure is accordingly transmitted through the passages 40 tb thespace 39 where it 50 acts eifectually upon both o'f the packing rings tion is produced between\the two rings 34 and-35 on the one hand and the cylinder I0 on the other hand,` and this increases the-emciency of the de- 55 vice as a shock "absorber or damper. It will be realized'that this mechanical friction is produced entirely as a result of the fluid friction induced by the flutter valve 44, and as a consequence its effeci; gradually'diminishes as a stateof equilibrium 60 is reached. This is considered to be advantageous as it enables .each strutof an aircraft to assume its position of equilibrium in a gentle but per'- fectly freemanner, and thus ensuresthat the machine will always be correctly poised but, when The ring carries out the additional function of preventing air from the atmosphere entering the strut down the space between the tubes I4 and I0 and so entering the space 41 where there is a tendency for a vacuum kto form on the extension stroke. i

f 'I'he pipes 23 and 24`enable exactly th-correct quantity of liquid to be'inserted when the strut is installed fand'also when it subsequently undergoes servicing and overhaul. For this purpose all air pressure is released through the air valve 21. 'I'he latter is then closed andthe two plugs 25 and 26 are removed. The strut is then compressed to its'full extent and oil is pumped in through one of the tubes, say 23. When the level'reaches therbottom of the two tubes, the strut, of course, being maintained in a substantially vertical position, the oil'is forced up the tube 24 and the two plugs 25 and 26 are then replaced. Air or other ,suitable gas is pumped in through the valve 21 until ther requisite pressure is reached and the strut is then ready for use. It will be observed that the interior of the cylinder tube I0 and the packing rings are always maintained in an eiciently lubricated condition, as the oil or other working liquid is forced under pressure within the space 39` during each retracting movement of the plunger tube.

It will be seen in Figure 2 that in addition to its simple construction, the improved strut canl readily be arranged so that there are practically no projecting partsand this is extremely benecial in the case'ofv struts utilized in aircraft as the aerodynamic drag is consequently reducible to a comparatively small value. n l

In general arrangementthe strut shown'in`Figures 4 and 5 isthe same as that in the previous examplethe upper end. of the plunger tube I4v being closed by,` a combined plug and rxing bracket 2|, while the lower end of the cylinder tube III` is closed by a similar fitting II. The

` upper ork inner end of thevcylinder tube' I0 is however turned inwardly at 55, its 4inwardly facing edge being conveniently ofA octagon'al shape as will be seen from Figure 5. An anti-friction ysleeve 56 is mounted within the lcylinder, tube I0 and h as its external surface 5 1 of Ycircular cross-v v lends itself to the nttingthereto of anydesired external part, Without danger of damage to any packing or sliding surfaces.

At its inner end the plunger tube I4 is turned inwardly as indicated at 59, the shoulder so formed conveniently having a circular aperture through which passes the stem of a tubular bolt 60, the head 6I of which serves to clamp the turned-in portion 59 in firm engagement with a fiat ring 62, which in turn bears upon the plunger head I6. An internally screw-threaded cap 63 serves as a nut for tightening up these parts, andis provided with a ange 64 which engages the lower surface of the plunger head I6 in a iiuid-tight manner owing to the'provision oi a packing ring 55. A packing 12 is placed between the plunger head I5 and the turned-in edge 59 of the tube I4. The

lower part 66 of the cap 63 also forms a housing for a flutter valve 44, which latter is permitted a limited axial travel between the parts 66 and the lower end of the bolt 60. The flutter valve 44 operates exactly as in the construction shown in Figure 1, and the chamber within which it is accommodated communicates by way of radial passages 5l with an annular clearance space 83 between the cap 63 and the plunger head I5, the latterbeing provided with radial' passages 40 'as before in order that the high pressure side of the flutter valve 44 may be automatically connected with the space 39 between a pair of packing rings 34 and`35, the peripheral portions of which are directed towards one another.

The compression 'of the strut is ilnally limited by an annular rubber or other resilient buffer 23 as before described, but in the present example the maximum-retracting movement is governed by the engagement of an anti-friction sleeve 59 uponthe plunger with a spacing tube I0 which bears at itsupper end against the sleeve 55. In order to secure a resilient action the sleeve 63 is sidable relative tothe plunger tube I4, and is flanged radially inwards at its lower end so as to provide a surfacewhich bears against a resilient ring 1I forming part of the plunger head.

It will be understood that the invention is not limited to the example shown,A as various other constructions may be evolved'having Vthe novel features now claimed. Moreover, it willbe appreciated that where strutsfaccording to the present invention are required to operate satisfactorily regardless of their orientation in relation to the vertical, a freely mounted piston member (not shown)` maybe incorporated as described in copending UnitedKingdom patent application No. 22936of 1935, this piston serving as a movable partition separating the liquid from the compressed gas Vwithin the` plunger tube I4. Fur- .f ther, by suitable modification of the plugs I I and 2I serving as the end boundaries of the space containing the operating liquid and gas, one or each of these plugs `may be disposed intermediate the endsA of the tube which it serves to close, thus enabling strut units of any desired length to be constructed irrespective of the volume required by the working iiuids i. e. the liquid and the com-l pressed gas, in order `to obtain the desired load capacity and rate of compression for a given stroke.

What we claim*y isz- 1i An oleo-pneumatic shock absorbing strut comprising a cylinder tube, a plunger tube slidably mounted therein, a piston head on the inner end ofr the plunger ,"tube and having a cavity, both of said tubes having means to close their outer parts so as to enclose a working space of variablevolume, a flutter valve within a cavity formed in the piston head and adapted to move to either end of the cavity away from that side of the piston head which is acted upon by ther greater fluid pressure, packing means around the piston head and comprising a pair of packing to l rings which are forced into contact with the cyl- 2. An oleo-pneumatic shock absorbing strut comprising a cylinder tube, a plunfger tube slidably mounted therein, a, piston headv onthe inner end of the plunger tube, said tubesy having ,means closing their Vouter ends so as to enclose wherebysaid space between therings is fed with l pressureV fluid fromrth'at side of vthepiston head having the greater pressure; f

3. An cleo-pneumatic shock absorbing strut comprising a cylinder tube, la plunger tube slidably mounted therein la pistonhead on the inner end of the plunger tube and having grooves, means closing theouter ends of both tubes so as to enclose a working space of variable volume,

' a cavity in the piston head, a flutter valve within the cavity and'adapted to move toeither end of the cavity away from that side of the piston head which is acted upon byl the greater fluid pressure packing means around the piston head and comprisingfrubber lrings whichare heldby their resilience in the grooves, av passageway connecting,k a space kbetween two of said rings with the cavity in the piston head so'tnat said greater fluid-pressurealways acts Ibetweenthe packing rings and forces them into contact withthe interior of the cylinder. y f

' 4. An toleo-pneumatic shock'absorbing strut as lclaimed in claim 3, wherein the piston head is v'formed 'witl'1'-a plurality of circumferential `grooves `each of which 'is approximately rectangular in 'radial section but has `the'r'naor axis of "its section arranged obliquely'with regard to the' axis of the pistonjhead, each' of the packing 'rings being correspondingly chamfered at its pe- 'y 'riphery so as'tc t against ythe linternal surface of the cylinder. y s

5. An .oleo-pneumatic shock .absorbing strut f comprising va cylinder tube, a. plungertube `slid-- ably mounted therein, a piston head' on the inner end of the plunger vtube and .means closing the outer ends of both tubes so as to. enclose a work ing space. ofv variable. volume, thel inner end of the plunger tube being .turned inwardlyl and being clamped axially to the piston head, said piston head having a cavity,a flutter valve within the cavity, and a plurality of packing-rings tted to the exterior ofv thel pistonhead, said piston head having a passageway connecting a space/between the packing ringswith the cavity/in. the piston head whereby said space vis, autcmatically connected with that side of lthepiston Ahead acted upon by the greater fluid pressure. f

6. An oleoepneumatic shock absorbingy strut comprising .a cylinder tube, a plunger tube slid-` ably mounted therein, a piston head on the inner end of the plunger tube, means closing the outer ends of both tubes so as toenclose a working space of variable volume, a cavity formed in the piston head, a Hutter valve therein, a plurality ci' packing, rings iltted to the exterior of the pistonl'ghead, a passagewayconnecting a space betweenthe packing rings with the cavity in the piston headwhereby said space is automatically connectedwith that side of the piston head acted upon by the greater iluid pressure, said cylinder tubebeing turned inwardly at its innerv end to engage slidably with the plunger tube. n

7. An oleo-pneumatic shock absorbing strut as claimed inV claim 6 in .which the inwardly turned part. of the'. cylinder tube is non-circular in shape and the corresponding part ofthe exteriorof the plunger tube is of similar shape in order tc. preventl relative rotaticn'between said plunger andV cylinder tubes. y

8. An oleo-pneumatic shock absorbing ,strut comprising a cylinder tube, a plunger vtube slidably mounted therein, a piston head on the inner end of the plunger tube and means closing the outer parts of. bothtubes so as to enclose a'working space ofy variable volume, a piston head upon the inner end or the plunger tube, packingmeans comprising a rpiurality of packing rings upon/the exteriorof'the piston head, a cavity within saic piston head bounded by end walls both having perforations, a iluttepplate mounted within said cavity and formed with a central aperture, said flutter plate being arranged to close `the perforationsy in one end wall of the cavity inthe piston head with the exception'of, a relatively small hole serving tc damp the extension of the strut by impeding the flow of fluid from the plunger tube to the cyiinder, and a passageway connectV ing lthe interior of the cavity in the piston hea-:i with a, space separating twoof the paci-:ing rings whereby the ilutterplate automatically places the space between Athe packing rings into communication with that side of the piston head which is variable volume, 'a plurality of packing rings suro rounding the 'piston head, a cavity in the piston head, agutter'valvewithin tn'e'cavity for damping the -fiow'oi fluid therethrough, a passageway from the'cavity inthe piston head to a space separating two of the packing vrings so thatr said space is put into communication with that side' of the piston'head having the greateriluid pres-` sure, an axiallyl slidable collar upon the piston head, and an annular cushion member interposed between the end surface of said collar and the corresponding part of the" piston head for forming a 'resilient stopt fo` limiting the extension movement of the plunger and cylinder tubes.

' yHENRY ONIONS.

PETER WARBORN THORNHILL. 

